1. Field of the Invention
The present invention relates to a vacuum regulating valve for regulating the pressure in an evacuated vacuum vessel for chemical reaction included in a physical or chemical machine or the like.
2. Description of the Related Art
A chemical process, such as an etching process, is carried out in a vacuum vessel included in a semiconductor device fabricating system. The vacuum vessel is evacuated at a negative pressure by a vacuum pump. A pressure regulating valve is placed in an external line connecting the vacuum vessel to the vacuum pump.
A pressure regulating valve disclosed in JP-A No. 8-178126 includes a valve casing having a valve chamber, provided with a first port having an axis parallel to the axis of the valve casing, a second port having an axis perpendicular to the axis of the valve casing, and a valve seat formed in an internal passage connecting the first and the second port; a disk-shaped valve element to be seated on the valve seat to close the pressure regulating valve; a valve operating unit for operating the valve element for opening and closing operations; a valve stem having one end connected to the valve element and the other end connected to the valve operating unit; and a bellows disposed behind the valve element so as to surround the valve stem in an air-tight fashion. One of the ports is connected to a vacuum vessel and the other is connected to a vacuum pump.
This prior art pressure regulating valve has a discharge characteristic indicated by a doted line in FIG. 5. In an initial stage of a valve opening operation, discharge rate increases sharply. Such a discharge characteristic may be due to the discharge of a fluid through a space around the valve element and a space between the bellows and the side wall of the valve casing upon the opening of the pressure regulating valve because the diameter of the valve element is comparatively small as compared with the inside diameter of the valve casing, and the space between the bellows and the side wall of the valve casing is wide. In most cases, it is desired to use a valve capable of discharging a fluid at a low discharge rate and of precisely controlling the discharge of the fluid in an initial stage of a valve opening operation because particles staying in a vacuum vessel and a passage connected to the vacuum vessel are liable to be stirred and raised and the control of the vacuum of the vacuum vessel is difficult if the vacuum vessel is evacuated rapidly.
It may be possible to discharge the fluid at a low discharge rate in the initial stage of the valve opening operation by using a bellows having a large outside diameter to narrow the space between the bellows and the side wall of the valve casing. However, the bellows extends and contracts according to the movement of the valve element, and the bellows vibrates when the same extends and contracts. Consequently, it is possible that the bellows wears rubbing against the side wall of the valve casing and produce dust. Therefore, a sufficiently wide space must be formed between the outer surface of the bellows and the side wall of the valve casing.
A valve disclosed in JP-A No. 9-133238 is provided with a cylindrical member, namely, a dust-adhesion preventing member, attached to a valve element and surrounding a bellows. This prior art valve has a valve casing provided with a first port having an axis parallel to the axis of the valve casing and connected to a vacuum pump, and a second port formed in the side wall of the valve casing and connected to a vacuum vessel. When the vacuum vessel is evacuated, the dust-adhesion preventing member prevents reaction products produced in the vacuum vessel and sucked through the second port into the valve from impinging on and sticking to the outer surface of the bellows. Therefore, the dust-adhesion preventing member is disposed nearer to the bellows than to the side surface of the valve casing to enhance the dust-adhesion preventing effect of the dust-adhesion preventing member. Consequently, a wide space is formed between the dust-adhesion preventing member and the side wall of the valve casing. Therefore, the valve does not have a function capable of preventing the flow of a fluid into the space between the dust-adhesion preventing member and the bellows when the vacuum vessel is evacuated. The diameter of a valve chamber formed in the valve casing and containing the bellows and the dust-adhesion preventing member is not uniform, and end part of the valve chamber is reduced. Therefore, the dust-adhesion member must be prevented from colliding against a wall defining the reduce part of the valve chamber at the end of the opening stroke of the valve element and hence the dust-adhesion preventing member needs to be spaced sufficiently apart from the side wall of the valve casing.